Road vehicles are known which include driven rear axles. The rear axle includes a crown wheel and pinion and differential. The pinion is driven via a drive shaft or the like connected to a prime mover such as an engine. The pinion and drive shaft rotate about a longitudinal axis of the vehicle. The pinion together with the crown wheel enables the crown wheel to rotate about an axis which is laterally orientated relative to the vehicle. The crown wheel drives a differential mechanism which drives a right and left hand drive shaft (as known as half shafts). The right hand drive shaft drives a right hand wheel rotatably mounted relative to the axle and the left hand drive shaft drives a left hand wheel rotatably mounted to the axle. In this way the vehicle can be driven over the ground. The crown wheel, pinion and differential assembly require cooling and lubrication and a fluid, typically an oil will perform this dual function. Under arduous operating conditions a greater amount of fluid is required, but under less arduous operating conditions less fluid is required. However, for convenience prior art axle assemblies have been filled with a predetermined amount of oil which is in permanent circulation around the crown wheel pinion and differential assembly.
This results in reduced efficiency, because under less arduous conditions more oil is circulating around the crown wheel pinion and differential than is required for lubrication and cooling purposes and this excess oil is therefore churned by the gears of the crown wheel, pinion and differential resulting in “churning” losses, i.e. power being wasted as it churns the oil.
In order to overcome this problem, it is known to raise the oil level around the crown wheel when operating under arduous conditions and to lower the oil level around the crown wheel when operating in less arduous conditions.
U.S. Pat. No. 6,299,561 uses walls within an axle housing in conjunction with a valve to raise or lower the oil around the crown wheel.
EP1918613 uses a shield fitted around part of the crown wheel so that the oil level within the shield can be raised or lowered.
WO2010/106482, GB2471652 and EP2270364 all show systems where the oil level around the crown wheel can be reduced to reduce oil churning losses but can also be increased under arduous conditions.
However, the prior art systems are all only suitable for applying to new vehicles, i.e. the prior art systems must be “designed into” a new vehicle and are not suitable for fitting to vehicles which are already in use because of the significant redesign and remanufacture required.
Since the current vehicle parc (i.e. the current population of vehicles on the road) will be used for several years to come, then a system for reducing the oil churning losses in such vehicles is required.